Firstly, make sure that the computer is published on NoMachine Network. To do that, go to the NoMachine Settings > Network > Machine > click Connect. A Machine Id and Access Id will be shown. These details are required in order to connect to the computer over NoMachine Network. You should share them only with users you trust and to those who require access to the computer.

By default, when publishing a computer on Network, its availability is to any Network user ('Everybody') who knows its Machine Id and Access Id. This setting can be changed by clicking 'Change' next to 'Available to:'.

You don't have to log in to the Network service to change a computer's availability but if you do, in the field for "selected User Ids", you can insert User Ids from your User book by selecting them from the drop-down list. If there are none in the list it means you have not added any yet. Adding users to your User book is easy. Go back to Settings > Network > Users and add the User Ids of the users you know.

Remember, anyone who wishes to connect over Network to your computer must know its Machine Id and Access Id. If you limit access exclusively to your own User Id, no one will be able to access the computer even if the Machine Id and Access Id are known.